The invention relates to the field of optical waveguide crossings.
In constructing integrated optical circuits, space constraints and the desire to operate on multiple input waveguides necessitate waveguide crossings. It is crucial that the crossings be as efficient as possible. A typical application is optical switching, where a large number of inputs are directed to as many outputs, and crossing is necessary in order for each input to connect to every output. Simplicity of fabrication on small lengthscales means that the waveguides must actually intersect, and cannot simply pass over one another. Any additional three-dimensional structure adds considerable manufacturing difficulty.
FIG. 1 is a block diagram of a conventional crossing arrangement 100. In a perfect crossing arrangement, optical modes 102 propagate with 100% transmission (throughput) from an input waveguide 104 to an output waveguide 106 on the opposite side of a crossing intersection 108, with no reflection and with 0% transmission (crosstalk) to the crossing waveguide 110. The attainment of low crosstalk is especially important since it is generally difficult to separate two signals that have been mixed, whereas low transmission can be remedied by a simple amplifier.
Previous works on waveguide crossings have dealt with waveguides based on index confinement (sometimes thought of as total internal reflection, the most familiar example of which is the fiber-optic cable). High (but not perfect) throughput can be attained in such devices, but only in the limit of very long or short wavelengths compared to the waveguide size. It is desirable to achieve perfect crossings for any wavelength, including the case where the waveguide width is of the same order as the wavelength, which allows maximum miniaturization. The design of good crossings in conventional devices has been a matter of trial and error.